This invention relates to a high-frequency variable attenuator for use in a radio communication circuit such as a portable telephone set and, more particularly, to a high-frequency variable attenuator including an attenuating circuit for attenuating an input high-frequency signal in response to a control voltage to produce an output high-frequency signal and a reference voltage generating circuit for supplying the attenuating circuit with a reference voltage.
In the manner which is well known in the art, a high-frequency variable attenuator of the type is for use in a radio communication circuit such as a portable telephone set and is implemented by a monolithic microwave integrated circuit. The high-frequency variable attenuator variably attenuates an input high-frequency signal in response to a control voltage to produce an output high-frequency signal. More specifically, the high-frequency variable attenuator comprises an attenuating circuit and a reference voltage generating circuit. The reference voltage generating circuit generates a reference voltage to supply the attenuating circuit with the reference voltage. The attenuating circuit has an input terminal supplied with the input high-frequency signal, an output terminal for producing the output high-frequency signal, and a control terminal supplied with the control voltage. On the basis of the control voltage, the attenuating circuit attenuates the input high-frequency signal in reference with the reference voltage to produce the output high-frequency signal.
The attenuating circuit includes an attenuator field effect transistor The attenuator field effect transistor has a gate terminal connected to the control terminal through a first attenuator resistor, a drain terminal connected to the input terminal through a first attenuator capacitor, a source terminal connected to the output terminal through a second attenuator capacitor. The reference voltage is supplied to the drain terminal and the source terminal of the attenuator field effect transistor through second and third attenuator resistors, respectively. Each of the second and the third attenuator resistors has a high resistance value. In addition, each of the first and the second attenuator capacitors plays a role for cutting off a direct current (DC) component.
In the manner which will later be described in conjunction with FIG. 1. a conventional high-frequency variable attenuator comprises a conventional reference voltage generating circuit for generating, as the reference voltage, a fixed reference voltage. In other words, a difference voltage between the control voltage and the reference voltage varies in dependency on only the control voltage. That is, the difference voltage is determined in response to the control voltage alone so as to have a constant variation amount.
More specifically, the conventional reference voltage generating circuit has a power-supply terminal supplied with a power-supply voltage and a ground terminal supplied with a ground voltage. The reference voltage generating circuit consists of a resistive potential divider. The resistive potential divider comprises first and second divider resistors which are connected in series between the power supply terminal and the ground terminal. The reference voltage generating circuit has a reference connection node between the first and the second divider resistors that generates a divided voltage as the fixed reference voltage.
Inasmuch as the difference voltage between the control voltage and the fixed reference voltage is determined so as to have the constant variation amount in the manner which is described above, in a case where the field effect transistor has a fixed structure, the conventional high-frequency variable attenuator has an attenuation amount having a slope which is uniquely determined in the manner which will later be described in conjunction with FIG. 2. In addition, the conventional high-frequency variable attenuator is disadvantageous in that the attenuation amount has variations with respect to a threshold voltage of the attenuator field effect transistor in the manner which will later be also described in conjunction with FIG. 2.
Various high-frequency variable attenuators of the type are already known. By way of example, Japanese Unexamined Utility Model Publication Jikkai No. Sho 59-134,928 or JP-U 59-134928 discloses a variable attenuator which comprises a first transistor connected between an input terminal and an output terminal, a first series circuit and a second series circuit. The first series circuit is connected between the input terminal and a common line and consists of a first resistor and a second transistor. The second series circuit is connected between the output terminal and the common line and consists of a second resistor and a third transistor. By varying direct current biases supplied to the first through the third transistors, an attenuation amount between the input terminal and the output terminal is varied.
Japanese Unexamined Patent Publication (JP-A) Tokkai No. Sho. 63-312,708 or JP-A 63-312708 discloses a variable attenuator which comprises first through third field effect transistors connected to each other in a xcfx80-shape fashion, and a plurality of resistors or diodes connected to gates sources, and drains of the first through the third field effect transistors. In the variable attenuator, two different negative control voltages are controlled by a positive fixed power source and a positive controllable power source. In other words, the variable attenuator disclosed in the JP-A 63-312708 is a xcfx80-type attenuator which has an attenuation amount control terminal, a constant voltage supplying terminal, a high-frequency signal input terminal, and a high-frequency signal output terminal.
Furthermore, Japanese Unexamined Patent Publication (JP-A) Tokkai No. Hei 6-69,754 or JP-A 6-69754 discloses a constant-resistance type variable attenuator in which a tertiary cross modulation distortion characteristic is improved by varying the gate width of a plurality of FETs applying control voltage on a gate so as to constitute the variable attenuator. More specifically, the bridged T type variable attenuator consists of a first field effect transistor (FET), first and second resistors, and a second FET. The first FET has a drain and a source connected to an input terminal and an output terminal for a microwave signal, and a gate connected to a first control terminal. The first resistor has one end connected to the drain of the first FET while the second resistor has one end connected to the source of the first FET. The first and the second resistors have other end connected to a common connection point in common. The second FET has a drain connected to the common connection point. a source grounded, and a gate connected to a second control terminal. The gate width of the second FET is enlarged so that the gate width of the first FET differs from that of the second FET. The saturated voltage of drain current upon the impression of the gate voltage around pinch-off increases as compared with the source voltage of the first FET, and the distortion characteristic of the second FET is improved.
Japanese Unexamined Patent Publication (JP-A) Tokkai No. Hei 6-77,762 or JP-A 6-77762 discloses a variable attenuator by incorporating a field effect transistor (FET) improved in mutual modulation distortion characteristics in a microwave integrated circuit concerning the variable attenuator for preparing the variable attenuating amount of microwaves by applying a control voltage to the gate of the FET The variable attenuator disclosed in JP-A 6-77762 comprises a field effect transistor (FET) and a choke coil. The FET has a source grounded, a drain connected to a signal line connecting between an input and an output, a gate connected to a control voltage terminal. A power source terminal is connected through the choke coil to the drain of the FET. When the control voltage is close to a threshold voltage of the FET and when the power supply terminal is biased, the non-linearity of the FET is relaxed and the mutual modulation distortion characteristic can be improved
In addition, Japanese Unexamined Patent Publication (JP-A) Tokkai No. Hei 9-265,786 or JP-A 9-265786 discloses a semiconductor storage which is capable of preventing erroneous read-out and of securing stable circuit operation by providing a negative feedback amplifier circuit containing circuit elements related to output decision in a feedback loop in a gate voltage generation circuit. In JP-A 9-265786, the gate voltage generation circuit comprises a reference voltage generation circuit, first and second differential amplifier circuits, and first and second divider circuits. The reference voltage generation circuit generates a reference voltage. The first and the second differential amplifier circuits are activated in response to a supply of an activation signal. The first and the second differential amplifiers amplify receiving the supply of the reference voltage to produce first and second differential output voltages, respectively. The first divider circuit receives the supply of the first differential output voltage and feeds back a first feedback voltage to the first differential amplifier circuit. The second divider circuit receives the supply of the second differential output voltage and feeds back a second feedback voltage to the second differential amplifier circuit. By such a constitution, dispersion in threshold voltages of the circuit elements of the first and the second differential amplifier circuits and the first and the second divider circuits are canceled, and the first and the second feedback voltages become equal to the reference voltage. Further, even when a source voltage is fluctuated, gate voltages are generated without being affected by that.
Furthermore, Japanese Unexamined Patent Publication (JP-A) Tokkai No. Hei 10-242,812 or JP-A 10-242812 discloses a semiconductor circuit which is capable of setting impedance so as not be affected by the concentration of a diffusion layer or the like. The semiconductor circuit disclosed in JP-A 10-242812 is provided with first and second I-type NMOS transistors connected in series between a power supply voltage point and ground. The first NMOS transistor has a gate terminal connected to a point of a reference voltage lower than a power supply voltage. The second NMOS transistor has a gate terminal to which the power supply voltage is applied. The second NMOS transistor has a drain voltage which is nearly equal to the reference voltage and the second NMOS transistor is acted in a linear region. Thus, the second NMOS transistor is used for an impedance element and it is not required to provide a resistive element separately and the effect by the concentration of a diffusion layer and a power supply voltage is avoided.
Japanese Unexamined Patent Publication (JP-A) Tokkai No. Hei 11-168,395 or JP-A 11-168395 discloses a transmission power control circuit which is capable of preventing unrequired transmission in the case that communication is not established. The transmission power control circuit disclosed in JP-A 11-168395 includes a variable attenuator, a control circuit, first and third reference voltage circuits, and a reference voltage switching circuit. The reference voltage switching circuit has a function for switching the first through the third reference voltage circuits for the control circuit. The control circuit controls the attenuation amount of the variable attenuator so as to output transmission power corresponding to the reference voltage.
It is therefore an object of this invention to provide a high-frequency variable attenuator which is capable of arbitrarily setting a slope of an attenuation curve indicative of an attenuation amount of an attenuating circuit with respect to a control voltage supplied to the attenuating circuit.
It is another object of this invention to provide a high-frequency variable attenuator of the type described, which is capable of suitably setting the attenuation amount in the attenuating circuit by suitably correcting the attenuation curve using a reference voltage supplied from a reference voltage generating circuit although there are variations in a threshold voltage of a field effect transistor, ambient temperature, or the like.
Other objects of this invention will become clear as the description proceeds.
On describing the gist of an aspect of this invention, it is possible to be understood that a high-frequency variable attenuator variably attenuates an input high-frequency signal in response to a control voltage to produce an output high-frequency signal.
According to a first embodiment of this invention, the above-understood high-frequency variable attenuator comprises a reference voltage generating circuit for generating a controllable reference voltage in response to the control voltage. Connected to the reference voltage generating circuit, an attenuating circuit attenuates the input high-frequency signal on the basis of the control voltage in reference with the controllable reference voltage to produce the output high-frequency signal.
According to a second aspect of this invention, the above-understood high-frequency variable attenuator comprises an attenuating circuit for attenuating the input high-frequency signal on the basis of the control voltage in reference with a controllable reference voltage to produce the output high-frequency signal. The attenuating circuit includes an attenuator field effect transistor having a threshold voltage. Connected to the attenuating circuit, a reference voltage generating circuit generates the controllable reference voltage so as to correct variations in the threshold voltage.
According to a third aspect of this invention, the above-understood high-frequency variable attenuator comprises an attenuating circuit for attenuating the input high-frequency signal on the basis of the control voltage in reference with a controllable reference voltage to produce the output high-frequency signal. The attenuating circuit includes an attenuator field effect transistor having a threshold voltage. The attenuating circuit has an attenuation amount determined by the threshold voltage and a difference voltage between the control voltage and the controllable reference voltage. The attenuation amount is represented by an attenuation curve which depends on the control voltage and the threshold voltage. Connected to the attenuating circuit, a reference voltage generating circuit generates, in response to the control voltage, the controllable reference voltage so as to determine a slope of the attenuation curve.